Battery-powered electronic devices, devices that derive some or all of their operating power from a battery, are popular, widely available, and in widespread use. In large part, the ultimate value and marketability of such devices depend on reliable battery power. Thus, given the importance of reliable battery power, most battery-powered electronic devices provide some form of a battery ‘fuel gauge’. The fuel gauge, generated by ‘fuel gauging’, displays a suggestion or indication of a remaining energy or charge level of the battery. Accurate fuel gauging generally enhances the reliability and predictability of a battery powered device. In contrast, inaccurate fuel gauging can actually decrease the apparent reliability of the battery-powered device.
First and foremost, fuel gauging is employed to keep a user of the device apprised of a current or remaining battery charge level. In addition, fuel gauging sometimes can provide an indication of probable remaining operating time of the electronic device. Using information supplied by a displayed fuel gauge reading, the user can determine whether or not the battery is likely to have sufficient remaining energy for the task at hand. If the user determines that the battery lacks sufficient energy based on the fuel gauge reading, the user can either replace the battery or, in the case of a rechargeable battery, recharge the battery. Thus, fuel gauging helps the user avoid an inconvenient or detrimental loss of operational power during the task due to insufficient available battery energy.
In addition to providing information to the user, fuel gauging, or more precisely, data collected by the electronic device and used to generate and display the fuel gauge reading, is often used by the device to determine whether or not a predetermined cut-off point in a discharge profile of the battery has been reached. As used herein, the cut-off point is a point in the discharge cycle of the battery at which there is insufficient remaining available energy to meet a given voltage and/or current requirement of the device using the battery. By detecting if and when the cut-off point has been reached, the device can initiate a ‘soft shut-down’, among other things. Such a device-initiated soft shut-down can help to prevent or at least mitigate various inopportune or inconvenient consequences of an unexpected loss of adequate operational power at or near an ‘end of charge’ of the battery.
While accurate battery fuel gauging is useful for and enhances the reliability of battery powered electronic devices, implementing such fuel gauging is not a simple, straightforward task. As is well known in the art, a battery is a device that stores energy as chemical energy and, when called upon to release the stored energy, converts the chemical energy into electrical energy. The battery delivers the electrical energy to a circuit or device attached to the battery as an electric current and voltage. Unfortunately, there is no convenient, direct means of determining or measuring an existing or remaining energy or charge level stored in a given battery. Moreover, there is no direct, convenient way to determine how much of the stored chemical energy is actually available to the device.
Thus, fuel gauging must generally employ an indirect means to predict or infer the remaining charge level of a battery. In most cases, battery characteristics, such as battery voltage and/or battery current, as a function of time, are used to deduce the charge level. The measured data front monitoring one or more of these battery characteristics are then translated into a fuel gauge reading or result using a fuel gauge algorithm.
To emphasize further these points, consider the problem of determining a total available energy of a battery. The total available energy of a battery is how ninth energy a battery can deliver to the device during a complete discharge cycle. Some measure of total energy available is normally used for fuel gauging. The total available energy from the battery typically depends on a number of variables. In fact, the total available energy generally depends on a chemistry of the battery, a physical size of the battery, a manufacturer of the battery, and how the battery is used by the device (i.e., load conditions to which the battery is subjected). Ultimately, the total available energy from a given battery is determined by how much energy is actually extracted from the battery before the cut-off point is reached. Thus, without draining the battery, the total available energy from the battery is difficult to ascertain.
Returning again to the general problem of fuel gauging, there are essentially two main techniques or fuel gauge algorithms employed to perform furl gauging in battery powered electronic devices: energy monitoring and voltage slope monitoring. Energy monitoring, sometimes called current monitoring, relies on monitoring energy flowing into and out of the battery as a function of time. If a total available energy at a starting point is known, then by measuring energy flowing into and out of the battery, a net change in the total available energy can be computed. This technique has the advantage that it measures energy flow directly. The disadvantages of this technique include a need to know the total available energy at a starting point. Since total available energy is strongly dependent on battery chemistry, size, manufacturer, and dynamic load conditions during discharge, this technique is most often used with application specific battery packs. An application specific battery pack is a custom packaged battery designed for a specific application or device. The performance characteristics, including total available energy of application specific battery packs, can be tightly controlled by the device manufacturer to minimize expected errors in fuel gauging.
Voltage slope monitoring, on the other hand, monitors a change in battery voltage as a function of time (dv/dt). A measured value of voltage is then used to determine a remaining amount of energy. The effectiveness of voltage slope monitoring depends on having an accurate relationship between the measured voltage value and the remaining energy level. Typically this is done using a curve or look-up table that relates energy to voltage. The advantage of voltage slope monitoring is that it can be implemented easily in most devices. The disadvantages include the need to construct the curve or look-up table to accommodate a range of possible actual relationships between voltage and energy. At the very least, the curve or look-up table must take into account a range of energy capacities provided by different manufacturers of batteries of appropriate form factors that can be used in the device. The problem is further exacerbated in devices that can use batteries having any one of multiple different chemistries and made worse yet by the fact that the battery voltage at a given energy level is highly dependent on the load that the battery is encountering.
The ‘chemistry’ of the battery refers to the specific combination of electrolytes and electrode materials used in the battery to create and sustain chemical reactions within the battery that produce the electricity. A variety of different battery chemistries are currently commercially available including alkaline, high-drain alkaline, nickel-metal hydride (NiMH), nickel-cadmium (NiCd), and photo lithium or lithium-iron sulfide (Li—FeS2). Moreover, all of these chemistries are available in a variety of common battery sizes or form factors, including, but not limited to, an ‘AA’ size.
The relevance of the chemistry of a battery to fuel gauging is due to a direct relationship between the chemical reactions and the electrical characteristics of the battery. Essentially all measurable electrical characteristics of a given battery, including but not limited to, open-circuit voltage, loaded voltage, charge capacity, peak current and even re-chargeability, are a direct result of the specific chemical reactions taking place within the battery. The unique qualities of a battery's chemical reaction, such as reaction rate, reaction path, and reactants involved, are sometimes referred to collectively as the ‘reaction kinetics’ or simply ‘kinetics’ of the battery. The reaction kinetics of the battery dictate the electrical characteristics of the battery. Thus, any of the electrical characteristics of the battery that might be usefully monitored for fuel gauging will be directly affected by or depend on the battery chemistry.
For example, the open-circuit voltages at full charge, mid charge and at the cut-off point can and do differ from one chemistry to another. In addition, peak current levels and internal resistance levels differ among the various chemistries leading to different measured voltages when the batteries are placed under a load. Thus, a fuel gauging approach designed or ‘calibrated’ for one chemistry may not be particularly accurate or effective for another chemistry even when using the same form factor.
As if the problems associated with using indirect methods for determining battery charge level were not complicated enough by multiple battery chemistries and multiple manufacturers, the total energy capacity of a given battery of a given chemistry from a given manufacturer can and does vary depending on how the battery is used in a particular device and how a given user employs the device as noted above. For example, alkaline batteries can deliver significantly more total energy over the course of a discharge cycle when subjected to low average load conditions as opposed to high average load conditions. In fact, an alkaline battery may provide as little as one-tenth the total energy output under high load conditions than under low load conditions.
Furthermore, batteries of various chemistries behave differently when subjected to dynamic or changing high and/or low load conditions. The measured voltage of a battery is directly affected by the load condition to which the battery is subjected. For example, batteries exhibit a phenomenon known as voltage recovery when a low load condition follows a high load condition. The voltage recovery, in turn, can produce erroneous readings by the fuel-gauging algorithm of the device. Thus, a given ‘use model’ or model of how the battery is used by a given device and how the user of the device employs the device during the battery discharge cycle, can and does impact the accuracy of the battery fuel gauging performed by the device.
Accordingly, it would be advantageous to have fuel gauging for battery powered electronic devices, wherein the accuracy of the fuel gauging is less sensitive to the effects of differing use models in different electronic devices than conventional fuel gauging.